Janne Avela

The Stretch-Shortening Cycle A Model to Study Naturally Occurring Neuromuscular Fatigue

Neuromuscular fatigue has traditionally been examined using isolated forms of either isometric, concentric or eccentric actions. However, none of these actions are naturally occurring in human (or animal) ground locomotion. The basic muscle function is defined as the stretch-shortening cycle (SSC), where the preactivated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. As the SSC taxes the skeletal muscles very strongly mechanically, its influence on the reflex activation becomes apparent and very different from the isolated forms of muscle actions mentioned above. The ground contact phases of running, jumping and hopping etc. are examples of the SSC for leg extensor muscles; similar phases can also be found for the upper-body activities. Consequently, it is normal and expected that the fatigue phenomena should be explored during SSC activities.The fatigue responses of repeated SSC actions are very versatile and complex because the fatigue does not depend only on the metabolic loading, which is reportedly different among muscle actions. The complexity of SSC fatigue is well reflected by the recovery patterns of many neuromechanical parameters. The basic pattern of SSC fatigue response (e.g. when using the complete exhaustion model of hopping or jumping) is the bimodality showing an immediate reduction in performance during exercise, quick recovery within 1–2 hours, followed by a secondary reduction, which may often show the lowest values on the second day post-exercise when the symptoms of muscle soreness/damage are also greatest. The full recovery may take 4–8 days depending on the parameter and on the severity of exercise. Each subject may have their own time-dependent bimodality curve.Based on the reviewed literature, it is recommended that the fatigue protocol is ‘completely’ exhaustive to reduce the important influence of inter-subject variability in the fatigue responses. The bimodality concept is especially apparent for stretch reflex responses, measured either in passive or active conditions. Interestingly, the reflex responses follow parallel changes with some of the pure mechanical parameters, such as yielding of the braking force during an initial ground contact of running or hopping. The mechanism of SSC fatigue and especially the bimodal response of performance deterioration and its recovery are often difficult to explain. The immediate post-exercise reduction in most of the measured parameters and their partial recovery 1–2 hours post-exercise can be explained primarily to be due to metabolic fatigue induced by exercise. The secondary reduction in these parameters takes place when the muscle soreness is highest.The literature gives several suggestions including the possible structural damage of not only the extrafusal muscle fibres, but also the intrafusal ones. Temporary changes in structural proteins and muscle-tendon interaction may be related to the fatigue-induced force reduction. Neural adjustments in the supraspinal level could naturally be operative, although many studies quoted in this article emphasise more the influences of exhaustive SSC fatigue on the fusimotor-muscle spindle system. It is, however, still puzzling why the functional recovery lasts several days after the disappearance of muscle soreness. Unfortunately, this and many other possible mechanisms need more thorough testing in animal models provided that the SSC actions can be truly performed as they appear in normal human locomotion.

Changes in muscle activity with increasing running speed

Electromyographic (EMG) activity of the leg muscles and the ground reaction forces were recorded in 17 elite male middle-distance runners, who performed isometric maximal voluntary contractions (MVC) as well as running at different speeds. Electromyograms were recorded from the gluteus maximus, vastus lateralis, biceps femoris, gastrocnemius and tibialis anterior. The results indicated that the averaged EMG (aEMG) activities of all the muscles studied increased (P < 0.05) with increasing running speed, especially in the pre-contact and braking phases. At higher speeds, the aEMG activities of the gastrocnemius, vastus lateralis, biceps femoris and gluteus maximus exceeded 100% MVC in these same phases. These results suggest that maximal voluntary contractions cannot be used as an indicator of the full activation potential of human skeletal muscle. Furthermore, the present results suggest that increased pre-contact EMG potentiates the functional role of stretch reflexes, which subsequently increases tendomuscular stiffness and enhances force production in the braking and/or propulsive phases in running. Furthermore, a more powerful force production in the optimal direction for increasing running speed effectively requires increased EMG activity of the two-joint muscles (biceps femoris, rectus femoris and gastrocnemius) during the entire running cycle.

Effects of marathon running on running economy and kinematics

Abstract The present study was designed to investigate interactions between running economy and mechanics before, during, and after an individually run marathon. Seven experienced triathletes performed a 5-min submaximal running test on a treadmill at an individual constant marathon speed. Heart rate was monitored and the expired respiratory gas was analyzed. Blood samples were drawn to analyze serum creatine kinase activity (S-CK), skeletal troponin I (sTnI), and blood lactate (B-La). A video analysis was performed (200 frames · s−1) to investigate running mechanics. A kinematic arm was used to determine the external work of each subject. The results of the present study demonstrate that after the marathon, a standardized 5-min submaximal running test resulted in an increase in oxygen consumption, ventilation, and heart rate (P < 0.05), with a simultaneous decrease in the oxygen difference (%) between inspired and expired air, and respiratory exchange ratio (P < 0.05). B-La did not change during the marathon, while sTnI and S-CK values increased (P < 0.05), peaking 2 h and 2 days after the marathon, respectively. With regard to the running kinematics, a minor increase in stride frequency and a similar decrease in stride length were observed (P < 0.01). These results demonstrate clearly that weakened running economy cannot be explained by changes in running mechanics. Therefore, it is suggested that the increased physiological loading is due to several mechanisms: increased utilization of fat as an energy substrate, increased demands of body temperature regulation, and possible muscle damage.

Forefoot Strikers Exhibit Lower Running-Induced Knee Loading than Rearfoot Strikers

PURPOSE Knee pain and Achilles tendinopathies are the most common complaints among runners. The differences in the running mechanics may play an important role in the pathogenesis of lower limb overuse injuries. However, the effect of a runners foot strike pattern on the ankle and especially on the knee loading is poorly understood. The purpose of this study was to examine whether runners using a forefoot strike pattern exhibit a different lower limb loading profile than runners who use rearfoot strike pattern. METHODS Nineteen female athletes with a natural forefoot strike (FFS) pattern and pair-matched women with rearfoot strike (RFS) pattern (n = 19) underwent 3-D running analysis at 4 m·s⁻¹. Joint angles and moments, patellofemoral contact force and stresses, and Achilles tendon forces were analyzed and compared between groups. RESULTS FFS demonstrated lower patellofemoral contact force and stress compared with heel strikers (4.3 ± 1.2 vs 5.1 ± 1.1 body weight, P = 0.029, and 11.1 ± 2.9 vs 13.0 ± 2.8 MPa, P = 0.04). In addition, knee frontal plane moment was lower in the FFS compared with heel strikers (1.49 ± 0.51 vs 1.97 ± 0.66 N·m·kg⁻¹, P =0.015). At the ankle level, FFS showed higher plantarflexor moment (3.12 ± 0.40 vs 2.54 ± 0.37 N·m·kg⁻¹; P = 0.001) and Achilles tendon force (6.3 ± 0.8 vs 5.1 ± 1.3 body weight; P = 0.002) compared with RFS. CONCLUSIONS To our knowledge, this is the first study that shows differences in patellofemoral loading and knee frontal plane moment between FFS and RFS. FFS exhibit both lower patellofemoral stress and knee frontal plane moment than RFS, which may reduce the risk of running-related knee injuries. On the other hand, parallel increase in ankle plantarflexor and Achilles tendon loading may increase risk for ankle and foot injuries.

Reduced stretch reflex sensitivity and muscle stiffness after long-lasting stretch-shortening cycle exercise in humans

Abstract It has been suggested that during repeated long-term stretch-shortening cycle (SSC) exercise the decreased neuromuscular function may result partly from alterations in stiffness regulation. Therefore, interaction between the short latency stretch-reflex component (M1) and muscle stiffness and their influences on muscle performance were investigated before and after long lasting SSC exercise. The test protocol included various jumps on a sledge ergometer. The interpretation of the sensitivity of the reflex was based on the measurements of the patellar reflexes and the M1 reflex components. The peak muscle stiffness was measured indirectly and calculated as a coefficient of the changes in the Achilles tendon force and the muscle length. The fatigue protocol induced a marked impairment of the neuromuscular function in maximal SSC jumps. This was demonstrated by a 14.1%–17.7% (n.s. –P < 0.001) reduction in the mean eccentric forces and a 17.3%–31.8% (n.s. –P < 0.05) reduction in the corresponding M1 area under the electromyograms. Both of these methods of assessing the short latency reflex response showed a clear deterioration in the sensitivity of the reflex after fatigue (P < 0.05–0.001). This was also the case for the eccentric peak stiffness of the soleus muscle which declined immediately after fatigue by 5.4% to 7.1% (n.s. –P < 0.05) depending on the jump condition. The results observed would suggest that the modulation of neural input to the muscle was at least partly of reflex origin from the contracting muscle, and furthermore, that the reduced muscle stiffness which accompanied the decreased reflex sensitivity could have been partly responsible for the weakened muscle performance due to impaired utilization of elastic energy.

Effects of power training on muscle structure and neuromuscular performance

The present study examines changes in muscle structure and neuromuscular performance induced by 15 weeks of power training with explosive muscle actions. Twenty‐three subjects, including 10 controls, volunteered for the study. Muscle biopsies were obtained from the gastrocnemius muscle before and after the training period, while maximal voluntary isometric contractions (MVC) and drop jump tests were performed once every fifth week. No statistically significant improvements in MVC of the knee extensor (KE) and plantarflexor muscles were observed during the training period. However, the maximal rate of force development (RFD) of KE increased from 18 836±4282 to 25 443±8897 N (P<0.05) during the first 10 weeks of training. In addition, vertical jump height (vertical rise of the center of body mass) in the drop jump test increased significantly (P<0.01). Simultaneously, explosive force production of KE muscles measured as knee moment and power increased significantly; however, there was no significant change (P>0.05) in muscle activity (electromyography) of KE. The mean percentage for myosin heavy chain and titin isoforms, muscle fiber‐type distributions and areas were unchanged. The enhanced performance in jumping as a result of power training can be explained, in part, by some modification in the joint control strategy and/or increased RFD capabilities of the KE.

Excitability at the Motoneuron Pool and Motor Cortex Is Specifically Modulated in Lengthening Compared to Isometric Contractions

Neural control of muscle contraction seems to be unique during muscle lengthening. The present study aimed to determine the specific sites of modulatory control for lengthening compared with isometric contractions. We used stimulation of the motor cortex and corticospinal tract to observe changes at the spinal and cortical levels. Motor-evoked potentials (MEPs) and cervicomedullary MEPs (CMEPs) were evoked in biceps brachii and brachioradialis during maximal and submaximal lengthening and isometric contractions at the same elbow angle. Sizes of CMEPs and MEPs were lower in lengthening contractions for both muscles (by approximately 28 and approximately 16%, respectively; P < 0.01), but MEP-to-CMEP ratios increased (by approximately 21%; P < 0.05). These results indicate reduced excitability at the spinal level but enhanced motor cortical excitability for lengthening compared with isometric muscle contractions.

Leg stiffness modulation during exhaustive stretch-shortening cycle exercise

The present study examined the effects of muscle activity modulation on leg stiffness during an exhaustive stretch‐shortening cycle (SSC) exercise in eight male subjects. Reaction force, electromyography (EMG) of the soleus (Sol), gastrocnemius (Ga) and vastus lateralis (VL) muscles and sledge seat position were recorded during the SSC exercise, consisting of 100 maximal intermittent drop jumps followed by a continuous submaximal jumping until exhaustion, on a sledge apparatus. Metabolic loading was determined by measuring blood lactate (La). No change was found in leg stiffness during the maximal jumps, whereas the subsequent submaximal jumping induced a significant reduction by 27±12% (P<0.05). Leg stiffness was closely related to the EMG ratio between the braking and push‐off phases in Sol (r=0.81, P<0.05) and particularly in Ga (r=0.98, P<0.001) (but not in VL, r=0.64, NS) at the end of the submaximal jumping. Furthermore, the post‐exercise La was significantly associated with the EMG ratio at the end of the submaximal jumping in Sol (r=−0.88, P<0.01) and Ga (r=−0.98, P<0.001). These results indicate that activity modulation between the braking and push‐off phases in the triceps surae muscle, particularly in Ga, plays an important role in leg stiffness adjustments during fatiguing SSC exercise. It is suggested that efficient activity modulation (i.e. high EMG ratio) of the triceps surae muscle during an intensive fatiguing SSC exercise may postpone the exhaustion and development of metabolic fatigue.

Interaction between muscle stiffness and stretch reflex sensitivity after long‐term stretch‐shortening cycle exercise

The short latency stretch‐reflex component (M1) and its interactions with muscle stiffness and with muscle performance were investigated before and after long‐term stretch‐shortening cycle (SSC) exercise. Dramatic fatigue induced reduction in maximal SSC performance capability, and electromyographic activity was accompanied by a consistent decrease in the M1 reflex component and eccentric peak stiffness of the muscle. It can be suggested, therefore, that the decreased muscle performance is not simply a direct effect of central or peripheral fatigue, but is partly due to impairment of the ability to utilize stiffness‐related elastic energy.

Innervation zone shift at different levels of isometric contraction in the biceps brachii muscle

Experiments were carried out to examine whether innervation zone (IZ) location remains stable at different levels of isometric contraction in the biceps brachii muscle (BB), and to determine how the proximity of the IZ affects common surface electromyography (sEMG) parameters. Twelve subjects performed maximal (MVC) and submaximal voluntary isometric contractions at 10%, 20%, 30%, 40%, 50% and 75% of MVC. sEMG signals were recorded with a 13 rows x 5 columns grid of electrodes from the short head of BB. The IZ shifted in the proximal direction by up to 2.4 cm, depending upon the subject and electrode column. The mean shift of all the columns was 0.6+/-0.4 cm (10% vs. 100% MVC, P<0.001). This shift biased the average values of mean frequency (+21.8+/-9.9 Hz, P<0.001), root mean square (-0.16+/-0.15 mV, P<0.05) and conduction velocity (-1.15+/-0.93 m/s, P<0.01) in the channels immediately proximal to the IZ. The shift in IZ could be explained by shortening of the muscle fibers, and thus lengthening of the (distal) tendon due to increasing force. These results underline the importance of individual investigation of IZ locations before the placement of sEMG electrodes, even in isometric contractions.